Gardening Columns

Get Seeds Off to a Good Start

First published on garden.org on February 28, 2008, by Suzanne DeJohn

Whether you are a novice or an old hand at starting your own vegetable and flower seeds indoors, it's helpful to understand the science behind seed germination. Just what does a seed need to germinate and grow into a vigorous, healthy seedling? Once you understand these needs, you'll be able to provide your seeds and seedlings with the best possible conditions.

What Seeds Need to GerminateWater. Water softens the protective seed coat, then is absorbed into the internal tissues. There, it causes chemical changes within the seed, and soon the tiny dormant embryo begins to grow.

Oxygen. Just like animals, plants respire -- that is, they consume oxygen as they metabolize food to create energy for growth. Because germinating seeds are so small and growing so quickly, they require an abundant, consistent supply of oxygen.

Warmth. Most seeds germinate best at slightly warmer than room temperature.

Seeds don't need fertilizer to germinate. A seed uses the energy from its food storage structure, the cotyledon, to fuel its initial growth. Most seeds don't need light to germinate; if they do the seed packet should tell you so.

Making Sure Germination Needs are Met1. Use a purchased seed-starting mix. These soilless mixes provide both good drainage and water-holding capacity. They're also sterile, helping minimize disease problems on vulnerable seedlings.

2. Choose containers with drainage holes. Plastic six-packs and flats are good choices and can be reused year after year. Biodegradable pots are fine, too.

3. Plant seeds at the proper depth. The seed packet should give this information. The rule of thumb is to plant the seed two to three times as deep as it is wide. Tiny seeds should be barely covered by soil mix, while squash seeds should be sown about an inch deep. (Note that some seeds need light to germinate, and should not be covered with soil but merely pressed into the soil surface. The seed packet will tell you this.)

4. Keep seed-starting mix evenly moist but not saturated. Think of it as a damp sponge that contains both water and air.

5. Place seed trays in a warm location -- the top of the refrigerator is often a good spot.

After Germination
Check pots every day. As soon as seedlings emerge:

1. Place pots in a bright location. The young plants need light to develop the green chlorophyll they need for photosynthesis. A sunny window will do but supplemental fluorescent lights will give you the best results. Suspend the lights just an inch or two over the tops of the plants. Light intensity diminishes over distance, and you want your seedlings to get as much light as possible.

2. Choose a slightly cooler location. You'll get sturdier, stockier seedlings at temperatures in the high 60s.

3. Begin fertilizing weekly with a soluble fertilizer once the seedling has one or two sets of leaves. Dilute fertilizer to half the recommended strength. Organic fertilizers are a good choice since they provide a range of nutrients, including micronutrients.

Thinning
You'll likely lose some seedlings to drying out, disease, or some unknown culprit. So always plant at least two seeds per six-pack cell or small container. Once seedlings have two sets of leaves, it's time to thin -- the hardest part of seed-starting for most gardeners. You want one seedling per pot, so you need to choose one plant and remove the other. Choose the healthiest, strongest-looking seedling, which isn't necessarily the tallest. Then snip off other seedlings at the soil line and discard them. Although it's possible to gently pry extra seedings and replant in a new pot, you risk damaging the remaining plant. Be realistic: How many tomato plants can you really fit in your garden?

Damping Off
If tiny seedlings look fine one day, then flop over the next (and you're sure they aren't wilted from dry soil), then you may be seeing what's called "damping off." This is a disease problem caused by several soil-borne fungi. They attack seedlings at the soil line, constricting the stems and causing seedlings to topple over. Once a seedling is affected, there's nothing you can do to save it. Using sterile seed-starting mix and running a fan on low to improve air circulation are good preventative measures.